<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>

    <style>
        h3{
            color: blue;
            font-size: 30px;
        }
        p{
            color: green;
            font-size: 30px;
        }
    </style>

</head>
<body>
    <h3>三级标题</h3>
    <p>hello world!</p>
</body>
</html>